Chemically sensitive sensor with lightly doped drains

ABSTRACT

A chemically sensitive sensor with a lightly doped region that affects an overlap capacitance between a gate and an electrode of the chemical sensitive sensor. The lightly doped region extends beneath and adjacent to a gate region of the chemical sensitive sensor. Modifying the gain of the chemically sensitive sensor is achieved by manipulating the lightly doped region under the electrodes.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/361,403 filed on Jul. 3, 2010, the content ofwhich is incorporated herein by reference in its entirety.

BACKGROUND

Electronic devices and components have found numerous applications inchemistry and biology (more generally, “life sciences”), especially fordetection and measurement of various chemical and biological reactionsand identification, detection and measurement of various compounds. Onesuch electronic device is referred to as an ion-sensitive field effecttransistor, often denoted in the relevant literature as an “ISFET” (orpHFET). ISFETs conventionally have been explored, primarily in theacademic and research community, to facilitate measurement of thehydrogen ion concentration of a solution (commonly denoted as “pH”).

More specifically, an ISFET is an impedance transformation device thatoperates in a manner similar to that of a MOSFET (Metal OxideSemiconductor Field Effect Transistor), and is particularly configuredto selectively measure ion activity in a solution (e.g., hydrogen ionsin the solution are the “analytes”). A detailed theory of operation ofan ISFET is given in “Thirty years of ISFETOLOGY: what happened in thepast 30 years and what may happen in the next 30 years,” P. Bergveld,Sens. Actuators, 88 (2003), pp. 1-20 (“Bergveld”), which publication ishereby incorporated herein by reference in its entirety.

Details of fabricating an ISFET using a conventional CMOS (ComplementaryMetal Oxide Semiconductor) process may be found in Rothberg, et al.,U.S. Patent Publication No. 2010/0301398, Rothberg, et al., U.S. PatentPublication No. 2010/0282617, and Rothberg et al, U.S. PatentPublication 2009/0026082; these patent publications are collectivelyreferred to as “Rothberg”, and are all incorporated herein by referencein their entirety. In addition to CMOS, however, biCMOS (i.e., bipolarand CMOS) processing may also be used, such as a process that wouldinclude a PMOS FET array with bipolar structures on the periphery.Alternatively, other technologies may be employed wherein a sensingelement can be made with a three-terminal devices in which a sensed ionleads to the development of a signal that controls one of the threeterminals; such technologies may also include, for example, GaAs andcarbon nanotube technologies.

Taking a CMOS example, a P-type ISFET fabrication is based on a P-typesilicon substrate, in which an N-type well forming a transistor “body”is formed. Highly doped P-type (P+) regions S and D, constituting asource and a drain of the ISFET, are formed within the N-type well. Ahighly doped N-type (N+) region B may also be formed within the N-typewell to provide a conductive body (or “bulk”) connection to the N-typewell. An oxide layer may be disposed above the source, drain and bodyconnection regions, through which openings are made to provideelectrical connections (via electrical conductors) to these regions. Apolysilicon gate may be formed above the oxide layer at a location abovea region of the N-type well, between the source and the drain. Becauseit is disposed between the polysilicon gate and the transistor body(i.e., the N-type well), the oxide layer often is referred to as the“gate oxide.”

Like a MOSFET, the operation of an ISFET is based on the modulation ofcharge concentration (and thus channel conductance) caused by a MOS(Metal-Oxide-Semiconductor) capacitance. This capacitance is constitutedby a polysilicon gate, a gate oxide and a region of the well (e.g.,N-type well) between the source and the drain. When a negative voltageis applied across the gate and source regions, a channel is created atthe interface of the region and the gate oxide by depleting this area ofelectrons. For an N-well, the channel would be a P-channel (andvice-versa). In the case of an N-well, the P-channel would extendbetween the source and the drain, and electric current is conductedthrough the P-channel when the gate-source potential is negative enoughto attract holes from the source into the channel. The gate-sourcepotential at which the channel begins to conduct current is referred toas the transistor's threshold voltage VTH (the transistor conducts whenVGS has an absolute value greater than the threshold voltage VTH). Thesource is so named because it is the source of the charge carriers(holes for a P-channel) that flow through the channel; similarly, thedrain is where the charge carriers leave the channel.

As described in Rothberg, an ISFET may be fabricated with a floatinggate structure, formed by coupling a polysilicon gate to multiple metallayers disposed within one or more additional oxide layers disposedabove the gate oxide. The floating gate structure is so named because itis electrically isolated from other conductors associated with theISFET; namely, it is sandwiched between the gate oxide and a passivationlayer that is disposed over a metal layer (e.g., top metal layer) of thefloating gage.

As further described in Rothberg, the ISFET passivation layerconstitutes an ion-sensitive membrane that gives rise to theion-sensitivity of the device. The presence of analytes such as ions inan analyte solution (i.e., a solution containing analytes (includingions) of interest or being tested for the presence of analytes ofinterest), in contact with the passivation layer, particularly in asensitive area that may lie above the floating gate structure, altersthe electrical characteristics of the ISFET so as to modulate a currentflowing through the channel between the source and the drain of theISFET. The passivation layer may comprise any one of a variety ofdifferent materials to facilitate sensitivity to particular ions; forexample, passivation layers comprising silicon nitride or siliconoxynitride, as well as metal oxides such as silicon, aluminum ortantalum oxides, generally provide sensitivity to hydrogen ionconcentration (pH) in an analyte solution, whereas passivation layerscomprising polyvinyl chloride containing valinomycin provide sensitivityto potassium ion concentration in an analyte solution. Materialssuitable for passivation layers and sensitive to other ions such assodium, silver, iron, bromine, iodine, calcium, and nitrate, forexample, are known, and passivation layers may comprise variousmaterials (e.g., metal oxides, metal nitrides, metal oxynitrides).Regarding the chemical reactions at the analyte solution/passivationlayer interface, the surface of a given material employed for thepassivation layer of the ISFET may include chemical groups that maydonate protons to or accept protons from the analyte solution, leavingat any given time negatively charged, positively charged, and neutralsites on the surface of the passivation layer at the interface with theanalyte solution.

With respect to ion sensitivity, an electric potential difference,commonly referred to as a “surface potential,” arises at thesolid/liquid interface of the passivation layer and the analyte solutionas a function of the ion concentration in the sensitive area due to achemical reaction (e.g., usually involving the dissociation of oxidesurface groups by the ions in the analyte solution in proximity to thesensitive area). This surface potential in turn affects the thresholdvoltage of the ISFET; thus, it is the threshold voltage of the ISFETthat varies with changes in ion concentration in the analyte solution inproximity to the sensitive area. As described in Rothberg, since thethreshold voltage V_(TH) of the ISFET is sensitive to ion concentration,the source voltage V_(S) provides a signal that is directly related tothe ion concentration in the analyte solution in proximity to thesensitive area of the ISFET.

Arrays of chemically-sensitive FETs (“chemFETs”), or more specificallyISFETs, may be used for monitoring reactions—including, for example,nucleic acid (e.g., DNA) sequencing reactions, based on monitoringanalytes present, generated or used during a reaction. More generally,arrays including large arrays of chemFETs may be employed to detect andmeasure static and/or dynamic amounts or concentrations of a variety ofanalytes (e.g., hydrogen ions, other ions, non-ionic molecules orcompounds, etc.) in a variety of chemical and/or biological processes(e.g., biological or chemical reactions, cell or tissue cultures ormonitoring, neural activity, nucleic acid sequencing, etc.) in whichvaluable information may be obtained based on such analyte measurements.Such chemFET arrays may be employed in methods that detect analytesand/or methods that monitor biological or chemical processes via changesin charge at the chemFET surface. Such use of ChemFET (or ISFET) arraysinvolves detection of analytes in solution and/or detection of change incharge bound to the chemFET surface (e.g. ISFET passivation layer).

Research concerning ISFET array fabrication is reported in thepublications “A large transistor-based sensor array chip for directextracellular imaging,” M. J. Milgrew, M. O. Riehle, and D. R. S.Cumming, Sensors and Actuators, B: Chemical, 111-112, (2005), pp.347-353, and “The development of scalable sensor arrays using standardCMOS technology,” M. J. Milgrew, P. A. Hammond, and D. R. S. Cumming,Sensors and Actuators, B: Chemical, 103, (2004), pp. 37-42, whichpublications are incorporated herein by reference and collectivelyreferred to hereafter as “Milgrew et al.” Descriptions of fabricatingand using ChemFET or ISFET arrays for chemical detection, includingdetection of ions in connection with DNA sequencing, are contained inRothberg. More specifically, Rothberg describes using a chemFET array(in particular ISFETs) for sequencing a nucleic acid involvingincorporating known nucleotides into a plurality of identical nucleicacids in a reaction chamber in contact with or capacitively coupled tochemFET, wherein the nucleic acids are bound to a single bead in thereaction chamber, and detecting a signal at the chemFET, whereindetection of the signal indicates release of one or more hydrogen ionsresulting from incorporation of the known nucleotide triphosphate intothe synthesized nucleic acid.

Ion-sensitive metal oxide field effect transistors (ISFET) are known.The chemical reactions sensed by these types of transistors result inelectrical signals that are very small in magnitude, and therefore mayrequire amplification by additional circuitry to provide signal gain sothe signal may be processed efficiently. The additional circuitry takesup real estate on the semiconductor substrate that may be used foradditional sensor elements instead of amplification circuitry. It wouldbe beneficial if a chemically sensitive sensor could have a modifiedgain to eliminate the need for the additional gain circuitry. Theinventor recognized the benefits of the following described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-D illustrate the building of a semiconductor substrateaccording to an embodiment of the present invention.

FIG. 2A-D illustrate a semiconductor doped to provide lightly dopeddrains according to an embodiment of the present invention.

FIG. 3 illustrates a diagram of the capacitance generated by therespective doping regions of a chemically sensitive sensor in anembodiment of the present invention.

FIG. 4 illustrates an exemplary structure of a chemically sensitivesensor according to an embodiment of the present invention.

FIG. 5 illustrates another exemplary structure of a chemically sensitivesensor according to an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments provide a chemically sensitive sensor with modified gain.The chemically sensitive sensor may include a microwell, a floating gateterminal, a drain terminal, a source terminal and a pair of dopedregions in a substrate. The microwell may accept a sample used in achemical reaction. The floating gate may be electrically coupled to agate electrode on the substrate. The drain terminal connection and thesource terminal connection may be electrical terminals on the chemicallysensitive sensor. The pair of doped regions in the substrate may eachinclude a lightly doped region and a highly doped region. Each oflightly doped regions may extend beneath the gate electrode on thesubstrate and each of the highly doped regions may extend to couplerespectively to the drain terminal and the source terminal.

Another embodiment may also provide a chemically sensitive sensor withmodified gain. The chemically sensitive senor may include a microwell, afloating gate terminal, a drain terminal, a source terminal, a pair ofelectrodes and a pair of doped regions in a substrate. The microwell mayaccept a sample used in a chemical reaction. The floating gate may beelectrically coupled to a gate electrode on the substrate. The drainterminal connection and the source terminal connection may be electricalterminals on the chemically sensitive sensor. The pair of electrodes maybe formed on the substrate and one electrode of the pair on either sideof the gate electrode. One of the pair of doped regions may include alightly doped region and a highly doped region, while the other one ofthe pair of doped regions may only include a highly doped region. Thelightly doped region may extend beneath a respective one of theelectrodes and the highly doped of each pair extends to couplerespectively with the drain terminal or the source terminal.

An embodiment may also provide a method for building a chemicallysensitive sensor according to an embodiment of the present invention.The method may forming a substrate with a first conductivity type ofdopant. An epitaxial layer may be built using the same conductivity typedopant used to form the substrate, but made less dense than the dopanton the substrate. An electrode layer may be formed on the epitaxiallayer formed from a different, second conductivity type of dopant thanthe first conductivity type of dopant used to form the substrate. Thedensity of dopant on both the electrode layer and the substrate may besimilar. The electrode layer may be masked and etched to produce gatesand electrodes. A first lightly doped region may be created adjacent toone of the electrodes using a multidirectional implant technique,wherein the first lightly doped region is formed from a dopant of aconductivity type opposite the epitaxial layer dopant. Diffusion nodesmay be produced that are self-aligned with the electrodes next to thegates, a first of said diffusion nodes contiguous with the first lightlydoped region, from a dopant of a conductivity type similar to the gates,electrodes, and lightly doped region. A floating gate electrode,electrodes above the diffusion area, and contacts for electrodes may beformed by alternating layers of insulation, dielectric, conductive andmetal layers.

FIGS. 1A-D illustrate the building of a semiconductor substrateaccording to an embodiment of the present invention. In this embodiment,the chemically sensitive sensor 100 may be fabricated on a polysiliconsubstrate 110 with a semiconductor doping, which in this example is a Por (P+)-type dopant, is formed as illustrated in FIG. 1A. As shown inFIG. 1B, an epitaxial layer (P-epi) 120 may be formed upon the P+ typesubstrate 110 from doping of similar conductivity type, i.e., P-type, asthe P+ type substrate 110, but at a lesser density. Of course, otherdoping such as N-type doping may be used.

The area where the charge coupled sensor cells are going to be formedmay be pre-doped at a dense doping level with a dopant havingconductivity type opposite, i.e., N+, to that of the substrate (P+) 110and epitaxial layer 120. In FIG. 1C, an N+ doping level 130 may be builtover the P-type epitaxial layer 120 and the P+ type substrate 110. TheN+ doping level 130 may be used within the charge coupled chemicalsensor area. Using a mask and etching operation, gate(s) 133 andelectrodes 134, 136 of the chemically sensitive sensor 100 may be formedin the pre-doped areas of the N+ doping levels 130 as shown in FIG. 1D.It will be understood by those of ordinary skill within the art that inthe described embodiment, the doping levels can be reversed.

The above disclosed embodiment describes the fabrication of a transistorthat may provide a first gain to any signal between the electrodes 134,136 based on a signal applied to the gate electrode 133. The gain of thechemically sensitive sensor 100 may be modified by inserting additionaldoping material at locations within the substrate 110 in locationsproximate to the electrodes 134, 136 or gate 133. The additional dopingmaterial may affect the capacitance of the transistor 100, whichconsequently modifies the gain of the transistor 100. The details of thegain modification will be explained in more detail with respect to FIG.3.

FIG. 2A illustrates an approximate location of additional doping in anepitaxial layer adjacent to a gate electrode according to an embodimentof the present invention. The device of FIG. 2A may have a substratestructure similar to that of FIG. 1C. The chemically sensitive sensor200 may include an epitaxial layer 220 of a first conductivity type ofdopant (i.e., P-type), a gate electrode 215 formed from a secondconductivity type of dopant (i.e., N-type), and lightly doped regions223, 225. The lightly doped regions 223, 225, also called lightly dopeddrains (LDD), may be of the same of conductivity type of dopant (i.e.,P-type) as the gate electrode 215. The gain of the chemically sensitivesensor 200 may be modified by the injection of a less dense dopant inthe lightly doped regions 223, 225 than the gate electrode 215. Thelightly doped regions 223, 225 may be formed by using, for example, amultidirectional injection technique to inject the less-dense dopantinto the lightly doped regions 223, 225 beneath the gate electrode 215.Of course, other techniques may also be used. Highly doped regions forthe source and drain terminals of the transistor may be added eitherbefore or after the lightly doped regions. FIG. 2B illustrates theplacement of diffusion nodes 223, 225, which may be highly dopedregions, for coupling to the source and drain terminals of thechemically sensitive sensor 220 with respect to the gate electrode 215and the lightly doped regions 223, 225. Using known doping techniques,the highly doped regions 223, 225 may be built with a high densitydopant of the same conductivity type of dopant as the gate electrode215.

FIGS. 2C and 2D illustrate an alternative embodiment for a chemicallysensitive sensor that has a modified doping. FIG. 2C illustrates achemically sensitive sensor 202 that in addition to a gate 210 may alsohave electrodes 214, 216. In FIG. 2C, the gates 210 and the electrodes214, 216 of the chemically sensitive sensor 202 can be masked to enablemultidirectional doping implants to be inserted adjacent to respectiveelectrodes 214, 216 to create lightly doped regions 227, 229, or lightlydoped drains (LDD). The LDD 227 and 229 may be formed from aconductivity type opposite the epitaxial layer, i.e., N+. In theillustrated embodiment, the LDD 227, 229 may be formed within thechemically sensitive sensor 202 beneath the electrodes 214, 216.Alternatively, the LDD 227, 229 may be implanted using alternateimplanting methods as are known in the art.

A photo resist layer can be used to mask desired areas and formdiffusion nodes 235, 237 that may be self aligned with the electrodes214, 216 next to the gates 210 and contiguous with the LDDs 227, 229.The diffusion areas 235 and 237 may be formed from a conductivity typesimilar to the gates 210, electrodes 214, 216 and LDDs 227, 229, andopposite from the conductivity type of the epitaxial layer 221. In FIG.2D, the N+ diffusion nodes 237 and 239 may be highly doped regions andmay be formed using any conventional technique known within thesemiconductor arts. In addition, other nodes may also be formed. Thelightly doped region 227, 229 may be doped at a dopant density levelthat is less than the dopant density level of the diffusion nodes 237and 239.

FIG. 3 is a diagram illustrating capacitances associated with therespective doping regions of a chemically sensitive sensor according toan embodiment of the present invention. The illustrated chemicallysensitive sensor 300 may include a gate electrode 384 and diffusionareas 391 and 395 that are built on an epitaxial layer 397. The dopingof the gate electrode 384 and diffusion areas 391, 395 is shown asN-type doping while the epitaxial layer 397 is shown as P-type. Ofcourse, the doping may be reversed. The diffusion areas 391 and 395 maybe doped at a high density. The diffusion area 391 may contact a sourceterminal of the chemically sensitive sensor 300 and the diffusion area395 may contact a drain terminal of the chemically sensitive sensor 300.Of course, the source terminal and the drain terminal may beinterchanged. The gate electrode 384 may connect to a floating gate (notshown) that will provide a signal. Depending upon the magnitude of thesignal from the floating gate, the channel 396 may be induced to conductaccording to known transistor principles. A signal on the draindiffusion area 395 may pass through the channel 396 to the sourcediffusion area 391. In addition, due to the fabrication of the sensor300, a gate-to-drain capacitance Cgd may be present between the gateelectrode 384 and the drain diffusion area 395. Similarly, agate-to-source capacitance Cgs may be present between the gate electrode384 and the source diffusion area 391. The values of these capacitancesCgd and Cgs may influence the signal gain of the sensor 300. Thecapacitance Cgd and Cgs may be the result of the junction area betweenthe doping of the diffusion areas 391 and 395 and the gate electrode384. A portion of the capacitance Cgd and Cgs may be attributed toparasitic capacitance at the junction of the diffusion area and the gateelectrode 384. The amount of parasitic capacitance may be adjusted byadding lightly doped regions 394 and 392 to the drain and sourcediffusion areas 395 and 391, respectively. The lightly doped regions 394and 392 may be doped at a lower density than the highly doped diffusionareas 391 and 395, but may add additional area at the junction of thediffusion areas 395 and 391 with the gate electrode 384. Due to theadditional area, additional parasitic capacitance Cpara1 and/or Cpara2may be present at the junction area. The density of the doping in thelightly doped areas will affect the amount of parasitic capacitanceCpara1 and/or Cpara2 created. Therefore, the total capacitance at thegate 384/drain diffusion area 395 may be approximately equal toCgd+Cpara1, while the total capacitance at the gate 384/source diffusionarea 391 may be approximately equal to Cgs+Cpara2.

The presence of parasitic capacitance values Cpara1 and/or Cpara2 mayalter the gain of the sensor 300. In a pixel having anchemically-sensitive sensor (e.g., an ISFET) and a row selectiontransistor, the pixel may be read out in a source follower configurationor in a common source configuration. The parasitic capacitance valuesmay affect the gain of the pixel in different ways depending on whichconfiguration is used. In the source follower configuration, a gain ofunity (1) is the maximum gain. Parasitic capacitors serve to attenuatethe gain. This is because the signal at the fluidic interface iscapacitively coupled to the floating gate of the chemically-sensitivesensor. The parasitic capacitors create a capacitive divider thatreduces the charge to voltage conversion that occurs at the gate.Therefore, in source follower configurations, eliminating LDD regionsand minimizing the parasitic capacitance provides the largest gain. Inthe common source configuration, it is desirable to control theparasitic capacitance in order to create negative feedback to establishsystematic gain values. Without any parasitic capacitance, the pixelwould operate open-loop during readout with a very large gain that isnot well controlled by process parameters. Therefore, because LDDregions can be well controlled and consistently matched between devices,the gain can be controlled by these established capacitance values. Inthe common source configuration, the most important overlap capacitor isCgd. The gain of the pixel is roughly equal to the double-layercapacitance divided by Cgd. By way of example, if the double layercapacitance is 3fF and the value of Cgd is 0.3fF, the gain of the pixelis approximately 10. This becomes a well controlled parameter when it iscontrolled by the LDD. To decrease the gain, Cgd is increased withlarger LDD extensions. To increase the gain, the LDD extensions arereduced to lower Cgd. It is not desirable to have Cgd so small that itis not well controlled. Therefore, controlling the LDD region to achievea gain in the range from 1 to 20 is desirable.

For example, in another embodiment, the lightly doped region 394associated with the drain diffusion area 395 may be eliminated, and onlythe lightly doped region 392 may be present in a sensor 300. In thiscase, the capacitance attributed to the source terminal 391 may be equalto Cgs+Cpara2, while the capacitance attributed to the drain terminal395 may only be equal to Cds. The gain of the sensor 300 in thisembodiment is different from the embodiment described above in whichboth parasitic capacitances Cpara 1 and Cpara 2 were present.Accordingly, the addition of lightly doped regions 392 and/or 394 may beused to alter the gain of the sensor 300, and thereby eliminateadditional circuitry that may be needed to amplify sensor 300 signal.

An exemplary illustration of a structure of a chemically sensitivesensor according to an embodiment of the present invention may be seenin FIG. 4. The chemically sensitive sensor 400 may include a microwellportion 401, a built-up portion 403, and a substrate portion 405. Themicrowell portion 401 may include a microwell 410 that may have apassivation layer, such as oxide layer 415, near the bottom of themicrowell 410. A chemical reaction to be detected may occur in themicrowell 410 and be detected by the a floating gate electrode 420 inthe built-up portion 403.

The built-up portion 403 may include the floating gate electrode 420built upon alternating layers of insulation and dielectric 432, 434,436, 461, 465, 469; conductive and metal layers 430, 452, 454, 456, 471,475 and 479; and a gate electrode 484. The substrate portion 405 mayinclude a substrate 499 that may be doped with a P+ dopant, an epitaxiallayer 497 that may also be a P-type dopant, and N+ doped regions 491(source) and 493 (drain). The N+ doped regions 491 (source) and 493(drain) may be highly doped regions, and the areas labeled 491′ and 493′may be lightly doped regions. The channel 494 may become conductivebased on the signal applied from floating gate 420 to the gate 484. Ofcourse, the doping in the substrate 405, in the regions 491 and 493, inthe epitaxial layer 497 and in gate 484 may be reversed.

The chemically sensitive sensor 400 may have, depending upon the mode ofoperation, a signal gain that is dependent, in part, upon the additionalparasitic capacitance provided by the lightly doped regions 491′ and/or493′ (if present). Embodiments of the chemically sensitive sensor 400can either be an NMOS or PMOS device, e.g., formed as a standard NMOS orPMOS device with a microwell above a floating gate. The microwellstructure 410 may contain an oxide or other material 415 that maytransport a chemical sample, e.g., a specific ion(s), to sense a chargeon the floating gate 420 of the chemically sensitive sensor 400. Thischarge transfer may then be read by read circuits (not shown) coupled tothe chemically sensitive sensor 400 and the amount of charge transfermay represent an amount of ions contained within the sample in themicrowell 410. It is in this way that each chemically sensitive sensor400 in an array can be used to detect local variations in the sample inthe microwell 410, for example, an ion concentration of a sample liquid,that is presented over an array (not shown) of chemically sensitivesensors 400.

Another exemplary structure of a chemically sensitive sensor accordingto another embodiment of the present invention will be described withreference to FIG. 5. The chemically sensitive sensor 500 may include amicrowell portion 501, a built-up portion 503, and a substrate portion505. The microwell portion 501 may include a microwell 510 that may havea passivation layer, such as oxide layer 515, near the bottom of themicrowell 510. The chemical reaction to be detected may occur in themicrowell 510 and be detected by the a floating gate electrode 520 inthe built-up portion 503.

The built-up portion 503 may include the floating gate electrode 520built upon alternating layers of insulation and dielectric 542, 545,546, 561, 565, 569 and conductive and metal layers 552, 555, 556, 571,574, 575, 577 and 579. A gate electrode 584 and contacts 564, 567 can beformed for the electrodes 581 and 585. The substrate portion 505 mayinclude a substrate 599 that may be doped with a P+ dopant, an epitaxiallayer 597 that may also be a P-type dopant, and N+ doped regions 591(source) and 595 (drain). Of course, the doping in the substrate 505 maybe reversed. The electrodes 581 and 585 may accumulate charge from thegate electrode 584 to facilitate confinement and isolation. The chargecoupling of the electrodes 581 and 585 with the gate electrode 584 mayform a pixel that may be placed into an array for addressable readout.The transistor gain may be increased by the charge transfer from thegate electrode 584 to the electrodes 581 and 585. Furthermore, thecharge transfer may be affected by the manipulation of the parasiticcapacitance as explained above with respect to FIG. 3, which alsoaffects the transistor gain. The parasitic capacitance may bemanipulated by the addition of lightly doped regions 591′ and/or 595′,which may be doped at a lesser density than the highly doped regions 591and 595. In addition, charge transfer may be also affected by the VRterminal 563 and the Tx terminal 585 that may act as barriers or wellsfor the charge packets.

It should be noted that embodiments using more or fewer metal andinsulation layers are envisioned and the foregoing embodiments aresimply exemplary examples. Additionally, the number of electrodes canvary greatly with differing embodiments. Following the formation of thefloating gate electrode, any additional electrodes and contacts for theelectrodes, the chemically sensitive sensors, including ions, can beformed by creating insulating or dielectric layers out of tetraethylorthosilicate (TEOS) or oxides and etching microwells above the floatinggate electrodes. The microwells can then have a passivation layer placedin at least the bottom of the microwell. Using the techniques describedbelow, chemicals and ions can be sensed using structures made fromprocesses similar to those described above. Varying embodiments of theforegoing structures are possible. For example, the gate electrodes maybe formed using single layer polysilicon. The structure can be madeusing N+ or P+ electrodes. It is also envisioned that embodiments usingsingle electrode polysilicon gap spacing makes it possible to chargecouple the electrodes at a small enough process nodes such as 0.13 umand below. Process nodes 0.13 um and below enable a charge coupledstructure to work in current CMOS processes. However, it should be notedthat that process nodes are not limited to being this small, and caneasily be larger. Embodiments employing surface channels, buriedchannels and using ion implantation to form channels stops using arealso envisioned.

Additionally, embodiments using buried charge transfer with multipleN-type implants to create desired potential profiles to avoid interfacestates and avoid flicker noise are envisioned.

It should be noted that the previous description provides simplyexemplary embodiments and that varying processes for the fabricating thechemical sensitive sensor disclosed herein will be readily apparent tothose skilled in the art. For example, the lightly doped drains could beformed before the gates and electrodes using masking techniques.Accordingly, the steps discussed herein do not need to be performed inany particular order and may be performed using any semiconductortechnique known within the art.

Several embodiments of the present invention are specificallyillustrated and described herein. However, it will be appreciated thatmodifications and variations of the present invention are covered by theabove teachings. In other instances, well-known operations, componentsand circuits have not been described in detail so as not to obscure theembodiments. It can be appreciated that the specific structural andfunctional details disclosed herein may be representative and do notnecessarily limit the scope of the embodiments.

Those skilled in the art may appreciate from the foregoing descriptionthat the present invention may be implemented in a variety of forms, andthat the various embodiments may be implemented alone or in combination.Therefore, while the embodiments of the present invention have beendescribed in connection with particular examples thereof, the true scopeof the embodiments and/or methods of the present invention should not beso limited since other modifications will become apparent to the skilledpractitioner upon a study of the drawings, specification, and followingclaims.

Various embodiments may be implemented using hardware elements, softwareelements, or a combination of both. Examples of hardware elements mayinclude processors, microprocessors, circuits, circuit elements (e.g.,transistors, resistors, capacitors, inductors, and so forth), integratedcircuits, application specific integrated circuits (ASIC), programmablelogic devices (PLD), digital signal processors (DSP), field programmablegate array (FPGA), logic gates, registers, semiconductor device, chips,microchips, chip sets, and so forth. Examples of software may includesoftware components, programs, applications, computer programs,application programs, system programs, machine programs, operatingsystem software, middleware, firmware, software modules, routines,subroutines, functions, methods, procedures, software interfaces,application program interfaces (API), instruction sets, computing code,computer code, code segments, computer code segments, words, values,symbols, or any combination thereof. Determining whether an embodimentis implemented using hardware elements and/or software elements may varyin accordance with any number of factors, such as desired computationalrate, power levels, heat tolerances, processing cycle budget, input datarates, output data rates, memory resources, data bus speeds and otherdesign or performance constraints.

Some embodiments may be implemented, for example, using acomputer-readable medium or article which may store an instruction or aset of instructions that, if executed by a machine, may cause themachine to perform a method and/or operations in accordance with theembodiments. Such a machine may include, for example, any suitableprocessing platform, computing platform, computing device, processingdevice, computing system, processing system, computer, processor, or thelike, and may be implemented using any suitable combination of hardwareand/or software. The computer-readable medium or article may include,for example, any suitable type of memory unit, memory device, memoryarticle, memory medium, storage device, storage article, storage mediumand/or storage unit, for example, memory, removable or non-removablemedia, erasable or non-erasable media, writeable or re-writeable media,digital or analog media, hard disk, floppy disk, Compact Disc Read OnlyMemory (CD-ROM), Compact Disc Recordable (CD-R), Compact DiscRewriteable (CD-RW), optical disk, magnetic media, magneto-opticalmedia, removable memory cards or disks, various types of DigitalVersatile Disc (DVD), a tape, a cassette, or the like. The instructionsmay include any suitable type of code, such as source code, compiledcode, interpreted code, executable code, static code, dynamic code,encrypted code, and the like, implemented using any suitable high-level,low-level, object-oriented, visual, compiled and/or interpretedprogramming language.

1. A chemically sensitive sensor, comprising: a floating gateelectrically coupled to a gate electrode on a substrate; a drainterminal connection; a source terminal connection; a pair of dopedregions in the substrate, each doped region including a lightly dopedregion and a highly doped region, wherein each of the lightly dopedregions extends beneath the gate electrode on the substrate and each ofthe highly doped regions extend to couple respectively to the drainterminal and the source terminal.
 2. The chemically sensitive sensor ofclaim 1, further comprising: a microwell to accept a sample.
 3. Thechemically sensitive sensor of claim 1, wherein the microwell has anoxide layer at the bottom of the well adjacent to the floating gate. 4.The chemically sensitive sensor of claim 1, wherein a parasiticcapacitance is present between the gate electrode and lightly dopedregions beneath.
 5. The chemically sensitive sensor of claim 1, whereingain of the chemically sensitive sensor is modified according to anamount of dopant used in the lightly doped region.
 6. The chemicallysensitive sensor of claim 1, wherein the lightly doped region is dopedat a dopant density level that is less than the dopant density level ofthe highly doped region.
 7. A chemically sensitive sensor, comprising: afloating gate electrically coupled to a gate electrode on a substrate; adrain terminal connection; a source terminal connection; a pair ofelectrodes formed on the substrate and one electrode of the pair oneither side of the gate electrode; a pair of doped regions in thesubstrate, one of the pair of doped regions including a lightly dopedregion and a highly doped region, wherein the lightly doped regionextends beneath a respective one of the electrodes and the highly dopedregion of each pair extends to couple respectively with the drainterminal and the source terminal.
 8. The chemically sensitive sensor ofclaim 7, further comprising: a microwell to accept biological material.9. The chemically sensitive sensor of claim 7, wherein the microwell hasan oxide layer at the bottom of the well adjacent to the floating gate.10. The chemically sensitive sensor of claim 1, wherein one of the pairof electrodes acts as a reference electrode and as a barrier or well forcharge packets.
 11. The chemically sensitive sensor of claim 1, whereinone of the pair of electrodes acts as a diffusion electrode andfacilitates charge packets.
 12. A chemically sensitive sensor,comprising: a floating gate electrically coupled to a gate electrode; asource formed with a lightly doped region and a highly doped region; anda drain formed with a lightly doped region and a highly doped region;wherein the lightly doped region of the source and the lightly dopedregion of the drain extend toward one another adjacent to the floatinggate into a channel region.
 13. The chemically sensitive sensor of claim12, the highly doped region of the source and the drain comprising: adopant concentration greater than the lightly doped region of the sourceand the drain.
 14. The chemically sensitive sensor of claim 12, whereinthe highly doped regions of the source and the drain extend away fromthe channel region and the gate region.
 15. The chemically sensitivesensor of claim 12, wherein the highly doped region of the source iscoupled to a metal contact.
 16. The chemically sensitive sensor of claim12, wherein the highly doped region of the drain is coupled to a metalcontact.
 17. The chemically sensitive sensor of claim 12, wherein thelightly doped region of the source is greater in volume than the lightlydoped region of the drain.
 18. The chemically sensitive sensor of claim12, wherein the lightly doped regions of the source and the drain causeincreased capacitance that limits gain of the chemical sensitive sensor.19. A method of making a chemically sensitive sensor, comprising:forming a substrate with a first conductivity type of dopant; buildingan epitaxial layer using the same conductivity type dopant used to formthe substrate, but made less dense than the dopant on the substrate;forming an electrode layer on the epitaxial layer formed from adifferent, second conductivity type of dopant than the firstconductivity type of dopant used to form the substrate, wherein thedensity of dopant on both the electrode layer and the substrate aresimilar; masking and etching the electrode layer to produce gates andelectrodes; creating a first lightly doped region adjacent to one of theelectrodes using a multidirectional implant technique, wherein the firstlightly doped region is formed from a dopant of a conductivity typeopposite the epitaxial layer dopant; producing diffusion nodes that areself-aligned with the electrodes next to the gates, a first of saiddiffusion nodes contiguous with the first lightly doped region, from adopant of a conductivity type similar to the gates, electrodes, andlightly doped region; and forming a floating gate electrode, electrodesabove the diffusion area, and contacts for electrodes by alternatinglayers of insulation, dielectric, conductive and metal layers.
 20. Themethod of claim 19, further comprising: creating a second lightly dopedregion adjacent to one of the electrodes using a multidirectionalimplant technique, wherein the second lightly doped region is formedfrom a dopant of a conductivity type opposite the epitaxial layerdopant.
 21. The method of claim 20, wherein a second of said diffusionnodes is contiguous with the second lightly doped region.
 22. The methodof claim 19, further comprising: forming an additional electrode on adiffusion area.
 23. The method of claim 19, further comprising: forminga microwell to hold a sample above the floating gate.